The present invention relates to pressure vessels and, in particular, it concerns a pressure vessel which has a thin unstressed metallic liner.
A number of different structures are known for containing fluids at elevated pressures. These structures are generally referred to as xe2x80x9cpressure vesselsxe2x80x9d. Requirements of safety, as well as attempts to reduce weight, have lead away from the use of simple metallic pressure vessels towards use of reinforced composite materials. In order to provide the required sealing characteristics, however, an additional inner liner must be provided. Hence the two principal types of pressure vessel currently in use both employ reinforced composite containers with either a seamless metallic or thermoplastic liner.
The use of a metallic liner generally provides a much longer operational life, better resistance to harsh environments, and better sealing characteristics than thermoplastic liners. The design of pressure vessels with metallic liners, however, presents its own particular problems, as will now be described.
Composite pressure vessels with metallic liners are manufactured by filament winding of fibers impregnated with resin matrix, together forming the composite material, around the metallic liner. The metal liner of these structures bears part of the applied internal pressure. In addition, incompatibility of the ranges of elastic behavior of the metal liner and composite material lead to residual compression stresses in the liner as a result of the xe2x80x9cproof pressurexe2x80x9d test (xe2x80x9cautofrittage phenomenonxe2x80x9d).
During subsequent application of internal pressure, the liner stretches and experiences corresponding tensile stress. In order to withstand these tension/compression stresses through repeated filling cycles over an extended period of usage, the liner must be relatively thick. Besides the clear implications of a thick liner for the weight of the vessel, the presence of a thick metallic layer also leads to safety problems.
In an effort to address these problems, attempts have been made to develop an unstressed metallic liner in which a thin metallic layer provides sealing properties while transferring all of the pressure load to the surrounding primary vessel. An example of such a structure is described in U.S. Pat. No. 5,292,027 to Lueke.
In order to avoid stressing of the liner, Lueke suggests a complicated xe2x80x9cherringbonexe2x80x9d pattern of parallelogram-like elements which provides undulations in two orthogonal directions. As a result, the liner readily stretches in any direction to conform to the deformation of the primary vessel.
The structure suggested by Lueke presents numerous problems of practical implementation. Firstly, the liner appears to contact the primary vessel at isolated points. Pressure applied to such a structure would not be effectively transferred to the primary vessel walls, and would probably result in immediate destruction of the herringbone pattern. Furthermore, the complicated structure would be extremely difficult to manufacture.
Another reference, U.S. Pat. No. 1,968,088 to Mekler, although less relevant than the Lueke reference, will be mentioned for its superficial similarity to one embodiment of the present invention. Mekler, in a patent filed before the introduction of reinforced composite materials into the art, describes a freely-expanding, corrugated protective liner for reaction vessels subjected to rapidly varying temperatures. The corrugations serve to prevent distortion and damage to the liner under extreme heat stress, while insulating the main vessel from the most extreme of the temperature variations. The reference does not address issues of performance under elevated pressure.
The structure described by Mekler is not suitable for use with fluids at elevated pressures. Since no solution is suggested for accommodating heat stress along the direction of elongation of the corrugations, it would appear that the liner must have a clearance from the ends of the primary vessel. As a result, the liner must be designed to bear a large proportion of any internal pressure. Additionally, no support is provided for the corrugations of the liner. Thus, if the liner was made from thin materials, the corrugated structure would rapidly deform and collapse under internal pressure. Finally, since this reference pre-dates the use of reinforced composite materials, Mekler clearly fails to teach any synergy between a liner structure and specific configurations of such composite materials.
Finally, reference is made to U.S. Pat. No. 3,446,385 to Ponemon which proposes the concept of a relatively unstressed metallic liner within a fiber glass reinforced load-bearing container. Several examples given (FIGS. 6 and 7 of the Ponemon reference) propose to achieve this using a bi-directional corrugated pattern, conceptually similar to, but less feasible than, that suggested by the aforementioned Lueke reference.
In the examples of FIGS. 3-5, Ponemon provides a liner structure which can accommodate stress in one direction only. With regard to stress in other directions, Ponemon suggests choosing a filament winding angle of slightly more than, or slightly less than, 54xc2x045xe2x80x2 which, he claims, generates deformations selectively in the required directions.
In fact, the solution proposed by Ponemon is based upon a fallacy and is non-operative. Specifically, Ponemon states: xe2x80x9cIt is known that winding filaments at a helical angle of 54xc2x045xe2x80x2 within a tolerance of 1 degree produces an elongation in the glass fibers that is oriented in only one direction. That is, if the winding helical angle is greater than 54xc2x045xe2x80x2, the elongation of the glass fibers will be only in the longitudinal direction of the axis of the vessel; whereas, if the helical winding angle is less than 54xc2x045xe2x80x2, the elongation of the glass fibers is in the transverse or hoop direction of the vessel.xe2x80x9d
The angle of 54xc2x045xe2x80x2 is indeed a well known winding angle: it is the angle at which axial and hoop deformations in a cylindrical pressure vessel are equal. However, the suggestion that pure axial or pure hoop deformations may be obtained by shifting one degree to either side of this value is completely fallacious. To the contrary, angles in the stated region to either side of the 54xc2x045xe2x80x2 angle will typically give very significant though non-equal variations in deformation in both axial and hoop directions. Furthermore, this value corresponds to an unstable singularity around which small variations in angle give rise to large, non-linear variations in deformation, making it an angle to be avoided when self-cancellation of stresses is sought.
There is therefore a need for pressure vessels with thin unstressed metallic liners which are convenient to produce and which effectively transfer applied pressure to the walls of the primary container.
The present invention is a pressure vessel which has a thin unstressed metallic liner.
According to the teachings of the present invention there is provided, a pressure vessel for containing a fluid at elevated pressure, the pressure vessel comprising: (a) a primary load-bearing container formed with at least one wall made of fiber-reinforced composite material, the shape of the primary container and the reinforcing directions of the fiber-reinforced composite material being configured such that, under a given change in the pressure of the contained fluid, a strain of the wall in a first direction is at least one order of magnitude less than a corresponding strain in a second direction perpendicular to the first direction; (b) an unstressed corrugated metallic liner positioned adjacent to at least part of an inner surface of the wall and forming part of a hermetic seal within the primary container, the liner having corrugations extending substantially parallel to the first direction such that the liner conforms to deformation of the wall in the second direction; and (c) a filler layer of elastic material interposed between the liner and the wall so as to substantially fill raised portions of the corrugations.
According to a further feature of the present invention, the filler forms a substantially contiguous layer between the liner and the inner surface of the wall.
According to a further feature of the present invention, the filler is substantially incompressible.
According to a further feature of the present invention, wherein the filler has a module of elasticity of less than about 104 kgxc2x7cmxe2x88x922.
According to a further feature of the present invention, the primary container has a cylindrical portion and dome-shaped end portions, the liner being deployed along substantially all of the inner surface of the cylindrical portion.
According to a further feature of the present invention, the corrugations form circumferential rings within the cylindrical portion.
According to a further feature of the present invention, the corrugations extend parallel to a central axis of the cylindrical portion.
According to a further feature of the present invention, the hermetic seal is completed by at least one additional metallic element, the additional metallic element being sealingly connected to the liner by welding.
According to a further feature of the present invention, the liner is made from metallic material having a given coefficient of thermal expansion, and wherein the internal structure of the fiber-reinforced composite material is configured so as to generate an effective coefficient of thermal expansion of the wall as measured along the first direction substantially equal to the given coefficient.
There is also provided according to the teachings of the present invention, a pressure vessel for containing a fluid at elevated pressure, the pressure vessel comprising: (a) an unstressed corrugated metallic liner forming part of a hermetic seal, the liner having corrugations extending substantially parallel to a first direction such that the liner accommodates deformation in a second direction perpendicular to the first direction, the liner having an external surface; (b) a filler layer of elastic material disposed as a substantially contiguous layer adjacent to the external surface of the liner and substantially filling the corrugations; and (c) a primary load-bearing container surrounding the hermetic seal, the primary container being formed with at least one wall made of fiber-reinforced composite material adjacent to the filler layer, wherein the shape of the primary container, the reinforcing directions of the fiber-reinforced composite material, and the mechanical properties of the filler layer are configured such that, under a given change in the pressure of the contained fluid, a strain caused in the liner parallel to the first direction is at least one order of magnitude less than a corresponding strain in the second direction.
There is also provided according to the teachings of the present invention, a method for producing a pressure vessel for containing a fluid at a given working pressure which is to be tested at a corresponding proof-test pressure, the method comprising: (a) providing a liner made from metallic material and configured so as to accommodate deformation in a first in-plane direction; and (b) constructing around the liner a primary container having a multiple layer wall made from fiber reinforced composite material, the thickness of the layers, the reinforcing directions of fibers within each layer, and the mechanical and physical properties of the fibers in each layer being chosen such that, when the liner is filled with fluid at the proof test pressure, deformation of the liner along a second in-plane direction perpendicular to the first in-plane direction is limited to within the elastic limit of the metallic material.
According to a further feature of the present invention, a layer of elastic filler material is provided between the liner and the primary container, the filler material being substantially incompressible, wherein the thickness of the layers, the reinforcing directions of fibers within each layer, and the mechanical and physical properties of the fibers in each layer of the primary container are chosen such that application of increased pressure within the liner generates a strain in the wall as measured in the second in-plane direction at least one order of magnitude less than the corresponding strain as measured in the first in-plane direction.